Site Search

There are several other fuel cells in the research labs and in development today. With this wide array of technologies and the myriad of applications, fuel cells are poised to revolutionize the way we think about and use energy today and in the future.

Different Types of Fuel Cells

There are
several other fuel cells in the research labs and in development today.
With this wide array of technologies and the myriad of applications, fuel
cells are poised to revolutionize the way we think about and use energy
today and in the future.

With the current debates
over energy, more people are aware of the benefits and potential
applications of fuel cells. However, few would be able to describe the
basic chemistry differentiating the application, power output and energy
efficiencies of the various types of fuel cells; much less the unique
challenges each face. Hence, a simple lesson in fuel cells is in order.

The Science

Fuel cells produce energy
without combustion by an electrochemical process using hydrogen fuel. A
fuel cell consists of two electrodes sandwiched around an electrolyte.
Hydrogen is fed to one of the electrodes. Oxygen (from the air) enters
the fuel cell through the other. Encouraged by a catalyst, the hydrogen
atom splits into a proton and an electron. The proton passes through
the electrolyte. The electrons create a separate current that can be
utilized before they are reunited with the hydrogen and oxygen to form
water molecules.

When a fuel cell system
is equipped with a &quot;fuel reformer," the fuel cell can utilize hydrogen
from a number of hydrocarbon fuels including natural gas, methanol,
propane, biomass, and gasoline. In principle, any hydrogen compound
will do. The emissions from reforming these various hydrocarbon fuels
would still be cleaner than those from a combustion process. It is also
possible to obtain hydrogen by separating water in an electrolyzer, or
by extracting it from a compound that contains no carbon, such as
ammonia or boron compounds.

Fuel Cell Types

Fuel cells are a family
of technologies. Fuel cell types are characterized by their
electrolytes and temperature of operation.

Proton exchange
membrane (PEM)
fuel cells have a solid polymer membrane as an electrolyte. Due to
membrane limitations, PEMs usually operate at low temperatures
(60-100°C/140-212°F), but new developments have produced higher
temperature PEMs (up to 200°C/392°F). Since platinum is the most
chemically active substance for low temperature hydrogen separation, it
is used as the catalyst. Hydrogen fuel is supplied as hydrogen gas or
is reformed from methanol, ethanol, natural gas or liquefied petroleum
gas and then fed into the fuel cell. The power range of existing PEMs is
about 50W to 150kW.

The
advantages of using PEM fuel cells include: 1) low weight and volume
with good power-to-weight ratio, 2) low temperature operation, so less
thermal wear to components, and 3) quick starts, with full power
available in minutes or less. These advantages make PEMs well-suited to
automotive and specialty vehicle applications such as scooters and
forklifts. Many on-road trials are providing information to make PEMs
competitive with internal combustion engines. Quick-starting PEMs can
also provide back-up power to telecommunications and other sites
requiring uninterrupted power supplies(UPS). PEMs additionally offer efficient
operation - up to 50% electrical efficiency for the fuel cell itself and
over 85% total efficiency when waste heat is captured for small-scale
space and water heating (combined heat and power, or CHP). Hundreds of
CHP and UPS PEM units have been deployed in demonstrations, and a number
of units are now available for sale.

Several challenges face
PEMs. Platinum catalysts are expensive and also subject to CO poisoning
from hydrocarbon fuels, so catalyst improvements, non-precious metal
catalysts and other alternatives are under investigation. Membranes more
resistant to chemical impurities are also being developed. Alternate
storage methods, such as metal hydrides and carbon nanostructures, may
address hydrogen storage limitations preventing fuel cell cars from
achieving typical driving range (300-400 miles/tank). Cold starts from
frozen internal water are improving - a US Department of Energy goal is
to achieve cold starts from -20°C (-4°F) in 30 seconds or less.

Direct methanol fuel
cells (DMFCs)
differ from PEMs because they use unreformed liquid methanol fuel rather
than hydrogen. DMFCs operate at slightly higher temperatures than PEMs
(50-120°C/120-248°F) and achieve around 40% efficiency. Since they are
refuelable and do not run down, DMFCs are directed toward small mobile
power applications such as laptops and cell phones, using replaceable
methanol cartridges at power ranges of 1-50 W. Many of the major
electronics companies are demonstrating miniature DMFCs powering their
equipment and smaller fuel cell companies are partnering with military
and communications contractors. The United Nations recently declared
that methanol cartridges were safe for shipment in airplane cargo
holds. Developers are currently addressing membrane corrosion, fuel
crossover and miniaturization challenges. DMFCs are poised for
widespread commercial availability in 2006, and some companies, such as
SFC Smart Fuel Cell AG, are selling products now.

The phosphoric acid
fuel cell (PAFC) is the fuel cell technology, with the greatest
experience in consumer applications. More than 200 PAFC fuel cell
systems are installed all over the world, providing power and useful
steam heat to hospitals, nursing homes, hotels, office buildings,
schools, utility power plants, an airport terminal, landfills and waste
water treatment plants. UTC Fuel Cells (formerly ONSI and International
Fuel Cells) paved the way for the technology, selling systems since the
early 1990's and recently reaching the milestone of more than one
billion kilowatt-hours of energy with its PureCell™ 200 power plant
solution. PAFCs use liquid phosphoric acid as an electrolyte with a
platinum catalyst. Anode and cathode reactions are similar to PEMs, but
operating temperatures are slightly higher (150-200°C/302-392°F) making
them more tolerant to reforming impurities. PAFCs use hydrocarbon
sources such as natural gas, propane or waste methane. PAFCs are
typically used for medium to large-scale stationary power generation,
attaining a 36-42% electrical efficiency and an overall 85% total
efficiency with co-generation of electricity and heat. The power range
of existing PAFCs is 25-250 kW. However, if several units are linked,
PAFCs can achieve a combined power output greater than 1 MW (an 11 MW
PAFC power plant is operating in Japan).

Fast-starting alkaline
fuel cells (AFCs) have been used by NASA to produce power and
drinking water for astronauts since the 1960s Gemini missions. AFCs
operate in an electrolyte solution of potassium hydroxide and can use a
variety of non-precious metal catalysts at operating temperatures of
23-250°C (74-482°F). Fueled by hydrogen gas, AFCs have a high chemical
reaction rate and offer an electrical efficiency of 60-70%. However,
AFCs are poisoned easily by small quantities of carbon dioxide, so they
are mostly used in controlled aerospace and underwater applications.
AFCs in Space Shuttle applications produce 12 kW of power.

Solid oxide fuel cells
(SOFCs) are one of the high temperature fuel cells, operating at
800-1000 °C (1472-1832°F) High temperature operation eliminates the
need for precious metal catalysts and can reduce cost by recycling the
waste heat from internal steam reformation of hydrocarbon fuels. SOFCs
are tolerant to CO poisoning, allowing CO derived from coal gas to also
be employed as source of fuel. These fuel cells use a solid ceramic
electrolyte and produce a power output of 2-100 kW and can attain 220
kW-300 kW when used in a SOFC/gas turbine hybrid system. Demonstrated
electrical efficiencies are 45-55%, with total efficiencies of 80-85%
with cogeneration of waste heat. SOFCs are well-suited for
medium-to-large scale, on-site power generation or CHP (hospitals,
hotels, universities), and are also being marketed for
telecommunications back up and as auxiliary power units (APUs) for
military vehicle on-board equipment.

Molten carbonate fuel
cells (MCFCs)
operate at 600-750°C (1112-1382°F) and use a molten alkali carbonate
mixture for an electrolyte. MCFCs typically range between 75-250 kW,
but when using combined units, have produced up to 5 MW of power.
Electrical efficiencies are 50-60%, with total efficiencies of 80-85%
with cogeneration of waste heat. To date, MCFCs have operated on
hydrogen, carbon monoxide, natural gas, propane, landfill gas, marine
diesel, and simulated coal gasification products.

The challenges to both
SOFC and MCFC development include slow start up, strong thermal
shielding requirements, and difficulty in developing durable materials
for the high temperature operating environment. Developers and the US
government (Solid State Energy Conversion Alliance) are also working on
lower cost, greater durability, low-temperature SOFCs (about 800°C), as
well as more powerful SOFC/gas turbine hybrids (1 MW or greater).
Current MCFC research focuses on reduction of size and cost, as well as
possible integration with gas turbines to increase performance.

There are several other
fuel cells in the research labs and in development today. With this
wide array of technologies and the myriad of applications, fuel cells
are poised to revolutionize the way we think about and use energy today
and in the future. For more information on fuel cells, please visit
www.fuelcells.org.

Post A Comment

Featured Product

Designed and manufactured in the U.S., the AllEarth Solar Tracker is a complete grid-tied, dual-axis solar electric system that produces up to 45% more electricity than fixed systems. The tracker uses GPS and wireless technology to follow the sun throughout the day for optimal energy production. It has an industry-leading 10 year warranty and 120 mph wind rating, superior snow shedding, and automatic high wind protection. Its simple, durable design and complete system pallet simplifies costly procurement and installation time. Contact us about becoming a dealer partner or purchasing an AllEarth Solar Tracker.